What if the spire, the rings of the Outlands, and the portals in the Cage are all part of a big explosion that took place / is in the process of taking place in the Outlands?
The rings of the Outlands represent the ripples of the explosion either frozen in time, or moving so slowly as to be unnoticeable by mortals?
The Spire is the plume at the centre of the explosion.
The Cage is either the source of the explosion, or the site of the explosion that was ejected / is being ejected from the Outlands.
The Dabus constantly plaster over the places in which Sigil threatens to tear apart. Now there are so many cracks that most mortals know them only as portals.
The reason why the gods cannot enter the Cage is not because of the Lady, but rather because the ever fragmenting city is moving beyond the reach of the gods.
Just some random thoughts at the end of the week?
If you're curious as to what caused the explosion? I heard Arianna Arakis say that it was caused by enough Spire butterflies detonating to create a singularity.